1. Field of the Invention
The present invention relates to an image forming apparatus that prints an image directly on an image that has been previously printed on a medium, a print data producing apparatus, and an image forming system.
2. Description of the Related Art
A conventional electro photographic image forming apparatus operates so that a developer image is first transferred onto a print medium and is then fixed by heat and pressure. When heat is applied to the developer image on the print medium, the printed image and the print medium can shrink by a certain percentage. For example, when a preceding image is formed on the print medium and then a following image smaller than the preceding image is printed directly on the preceding image, the following image may also shrink but probably by a different percentage from the preceding image. The resultant image printed on the print medium is not exactly the same as what appears on the monitor of, for example, of a personal computer (PC). This is due to the fact that the relation in size between the preceding image and the following image, which are printed on the print medium, is different from that between the preceding image and the following image, which appear on the monitor of, for example, a personal computer.
A conventional image forming apparatus employs a sensor or sensors incorporated therein for measuring the size of the print medium and/or the image printed on the print medium. The sensors measure the size of the print medium before an image is printed on the print medium, and then measure the size of the print medium and the printed image after the image has been printed, thereby determining their reduction ratios. The following image is printed with a substantially the same reduction ratio as the preceding image.
A sensor or sensors for measuring the size or dimension of images and print medium adds to the cost and the number of parts of the image forming apparatus.
Drawbacks due to shrinkage of print medium occur not only in a duplex printing but also in printing a special color image (e.g., white or transparent). Special color toners including white and transparent toners are designed such that special color toner does not mix with cyan, magenta, yellow, and black toners. A special color toner layer is laid on or under the cyan, magenta, yellow, and black images. Thus, in order to print the special color image, the special color image needs to be transferred and fixed in a first pass printing process or in the second pass printing process depending on the order of images are transferred and fixed and the order in which a plurality of image forming sections for printing the cyan, magenta, yellow, and black images.
With an image forming apparatus in which black, cyan, magenta, and yellow images are transferred onto the print medium in this order, a special color image (e.g., transparent) may be transferred over the uppermost of the black, cyan, magenta, and yellow images. If the special color image is to be printed as a background which is the lowermost of the black, cyan, magenta, and yellow images, only the special color image needs to be transferred and fixed before the black, cyan, magenta, and yellow images are transferred and fixed. When a plurality of images are to be transferred and fixed in a plurality of steps on a print medium, one or more of the images need to be magnified or reduced in size before printing, thereby minimizing a drawback resulting from shrinkage of the print medium due to heat during fixing.
The special color image needs to be produced in accordance with the black, cyan, magenta, and yellow images. In other words, the special color, black, cyan, magenta, and yellow images initially have the same size, and then some of the images needs to be magnified or reduced. When an image is reduced, the image is thinned or mottled. If the image is magnified or reduced in size using, for example, a dithering pattern in accordance with a desired output resolution, the dithering period tends to deviate from what it was.